Strange delivery numbers

Hello, hope somebody can help me.
Just today I have noticed that, in my last 2 deliveries, the numbers that usually point out the how many emails were delivered, indicate less emails than I have in my list.
In my specific case I have about 900 people in my list
but as you can see in the image below only 586 emails were delivered
https://bit.ly/1251DeT
please also note that I’m using a custom smtp server to send emails and that in “sending with…” section I set 90 emails each hours that should agree my server rules.

I hope you can help me out
thanks
g.

https://www.ads-software.com/extend/plugins/wysija-newsletters/